Within the meta-box/ing/sanitizer.php file, all the field types with a string sanitizer (buit-in WP sanitizing) are returning empty strings and values are not saved.
'email' => 'sanitize_email',
'file_input' => 'esc_url_raw',
'hidden' => 'sanitize_text_field',
'oembed' => 'esc_url_raw',
et...